An accurate estimate of formation pore pressure is a key requirement for the safe and economic drilling in overpressured sediments. Conventional methods of predicting pre-drill pore pressures are based on use of seismic velocities together with a velocity-to-pore-pressure transform, calibrated to offset well data (See, e.g., Sayers, C. M., Johnson, G. M. and Denyer, G., 2002, “Pre-drill Pore Pressure Prediction Using Seismic Data,” Geophysics, 67, pp. 1286-1292). However, these methods depend on the availability of accurate pre-drill seismic velocities.
A pre-drill estimate of formation pore pressures can be estimated either by using offset wells directly, or by using these to determine a velocity-to-pore-pressure transform, and then applying this transform to seismic velocities at the proposed well location. Examples of such transforms include the method of Eaton, which is described in “The Equation for Geopressure Prediction from Well Logs” SPE 5544 (Society of Petroleum Engineers of AIME, 1975), and that of Bowers, which is described in “Pore pressure estimation from velocity data: Accounting for pore-pressure mechanisms besides under compaction,” SPE Drilling and Completion (June 1995), pp. 89-95. These predictions can be updated while drilling the well, using Measurements While Drilling (MWD), Logging While Drilling (LWD), or other drilling data.
Previous studies based on x-ray diffraction (XRD) analysis of Gulf of Mexico data (Holbrook, 2002, “The primary controls over sediment compaction,” AAPG Memoir, 76) have suggested that transformation of the clay mineral Smectite into Illite may be associated with the onset of over-pressure (Dutta, N.C., 2002, “Geopressure prediction using seismic data: current status and the road ahead,” Geophysics, 67). This diagenetic process is primarily dependent upon potassium concentration and temperature, and is believed to occur within a relatively narrow temperature range (175±25° F.). It is typically characterized by a sigmoidal relationship between temperature and mineralogy indicators like grain density, with an inflection point occurring at the approximate Smectite-Illite conversion temperature (Lopez, J. L, Rappold, P. M., Ugueto, G. A., Wieseneck, J. B, Vu, C. K., 2004, “Integrated shared earth model: 3D pore-pressure prediction and uncertainty analysis,” The Leading Edge, 23, pp. 52-59).
FIG. 1 shows an exemplary diagram of an oilfield operation. Those skilled in the art will appreciate that the oilfield operation shown in FIG. 1 is provided for exemplary purposes only and accordingly should not be construed as limiting the scope of the invention. For example, the oilfield operation shown in FIG. 1 is a seafloor oilfield operation, but the oilfield operation may alternatively be a land oilfield operation or any other type of oilfield operation involved in the exploration, extraction, and/or production of fluids from a subterranean formation.
As shown in FIG. 1, a drilling rig (105) is configured to drill into a formation (e.g., a subterranean formation below a seafloor (115)) using a drill bit (not shown) coupled to the distal end of a drill string (125). Specifically, the drill bit is used to drill a borehole (130) extending to an area of interest (120). The area of interest (120) may be hydrocarbon, a mineral resource, or fluid targeted by an oilfield operation. Water depth may correspond to the vertical distance between the sea surface (110) and the seafloor (115). Subsurface vertical depth may correspond to the vertical distance between the sea surface (110) and the area of interest (120). Further, the subsurface (not shown) above the area of interest (120) may be referred to as overburden. The overburden may include soil and materials of varying densities.
When sediment of low permeability substance is buried or compacted, fluid may be trapped in pores within the resulting structure (i.e., within the low permeability substance itself and/or within substances beneath the low permeability substance (e.g., sand, etc.). Fluid trapped in this manner exerts pressure on the surrounding formation referred to as pore pressure. Formations in which pore pressure exceeds hydrostatic pressure at a given depth are referred to as overpressured.
When drilling in an overpressured formation, the mud weight (i.e., the weight of drilling fluids transmitted to the borehole) must be high enough to prevent the pore pressure from moving formation fluids into the borehole. In the worst case, formation fluids entering a borehole may result in loss of the well and/or injury to personnel operating the drilling rig. Accordingly, for safe and economic drilling, it is essential that the pore pressure be predicted (and monitored) with sufficient accuracy. In particular, it is beneficial to predict pore pressure pre-drill, i.e., either before any drilling has commenced and/or at a location that the drill bit has not yet reached.
Conventionally, pre-drill pore pressure prediction is based on the use of pre-drill seismic velocities and a velocity-to-pore pressure transform calibrated using offset well data (i.e., data from other wells near the drilling site). However, in some cases (e.g., when drilling under salt), conventional pre-drill pore pressure predictions may not be sufficiently accurate. Further discussion of conventional pre-drill pore pressure prediction techniques can be found in Sayers C M, Johnson G M, and Denyer G., 2002, “Pre-drill Pore Pressure Prediction Using Seismic Data,” Geophysics, 67, pp. 1286-1292.
Mud is used in oilfield operations to cool the drill bit, to transport cuttings generated by the oilfield operation to the surface, to prevent the influx of formation fluids into the borehole, and to stabilize the borehole. With respect to preventing the influx of formation fluids, the drilling operator must maintain the mud weight at or above the pore pressure. With respect to stabilizing the borehole, drilling operators adjust the mud weight (i.e., the density of the mud being used) to counter the tendency of the borehole to cave in. However, the drilling operator must be careful not to fracture the formation by using an excessively high mud weight.
Moreover, too high a mud weight may result in an unacceptably low drilling rate. Accordingly, the mud weight must be low enough to maintain an acceptable drilling rate and avoid fracturing the formation. In such cases, the allowable mud weight window (i.e., the range of allowable mud weights) may be small when drilling in overpressured formations. Specifically, the force exerted by the mud must fall within the range between the pore pressure (or the pressure to prevent a cave in, if higher than the pore pressure) and the pressure required to fracture the formation.
Further, when drilling in overpressured formations, the number of required casing strings (i.e., structural supports inserted into the borehole) may be increased. Specifically, if a sufficiently accurate pre-drill pore pressure prediction is not available, additional casing strings may be inserted prematurely, to avoid the possibility of well control problems (e.g. influx of formation fluids) and/or borehole failure. Prematurely inserting casing strings may delay the oilfield operation and/or reduce the size of the borehole and result in financial loss.